Various metal materials such as common steel, stainless steel, aluminum, aluminum alloy, copper, and galvanized steel are widely used in many fields, such as building materials and electronic devices, for their excellence in corrosion resistance and external appearance. When these metal materials are used as structural members, various components and so on, it is often the case that metal material plates need to be joined together or with other components or members. In this case, metal material plates are conventionally often joined by welding.
However, when metal material plates are joined by welding, there is a problem that weld marks are left on the surfaces of the welded metal material plates, which impairs excellent external appearance unique to metal material plates. Further, sheet-metal processing is required to remove weld marks or weld distortions. Such sheet-metal processing is unpopular among workers as well as peripheral neighboring residents because a lot of time and effort are required and working environment is worsened by noise generation etc.
For this reason, as a method for joining metal materials as an alternative to welding, a bonding method using an adhesive has received attention in recent years. The bonding method using an adhesive is advantageous in that the external appearance of metal materials is hardly impaired and therefore the above-described sheet-metal processing is not required. However, the surface of a metal material is generally often coated with a stable oxide layer. Particularly, stainless steel is coated with an oxide layer excellent in corrosion resistance but very low in affinity for an adhesive. Because of this, metal materials have a problem that they are poor in adhesion. The bonding method using an adhesive is thus disadvantageous in that an adhesive interface is poor in water resistance, and therefore when an adhesive joint between metal materials is exposed to a high-temperature and high-humidity environment, bond strength is significantly reduced in a short period of time.
The affinity of a metal material for an adhesive, especially for an epoxy-based adhesive can be improved by previously subjecting the surface of the metal material to activation treatment using an acid. For example, a method is known in which the surface of a stainless steel plate is treated with a mixed aqueous solution of sulfuric acid and oxalic acid. Further, a method is known in which an aluminum plate or an aluminum alloy plate is immersed in a phosphoric acid aqueous solution or a dichromic acid aqueous solution, or alternatively, an aluminum plate or an aluminum alloy plate is electrically anodized while being immersed in such an aqueous solution. These treatment methods are known to develop excellent adhesiveness, and are therefore practically used in, for example, an assembly process of an airplane.
However, the above-described method for activating the surface of a stainless steel plate by acid treatment has a problem that smut is generated on the surface of the stainless steel plate. This smut can be removed by treating the surface of the stainless steel plate with a mixed aqueous solution of dichromic acid and sulfuric acid. However, such desmutting treatment generates chromium-containing wastewater and is therefore strictly limited from the viewpoint of environmental destruction.
As a bonding method not requiring such desmutting treatment, a method has recently been tried, in which a primer is previously applied onto the surface of a stainless steel plate to form an organic thin film (primer layer) to improve adhesiveness. For example, Patent Literature 1 discloses a method to enhance the adhesiveness of the stainless steel plate. According to this method, the surface of a stainless steel plate is treated with an aqueous primer containing an acidic phosphoric acid ester and/or a salt thereof and water. Patent Literature 2 discloses a method in which the surface of a common steel plate or a stainless steel plate is treated with a silane-based coupling agent to improve its adhesiveness to a fluorine-based coating film.
It has been confirmed that the affinity of common steel plates, stainless steel plates, aluminum plates, and aluminum alloy plates for an epoxy-based adhesive is improved by such surface treatment using an acidic phosphoric acid ester or a silane-based coupling agent. However, such surface treatment methods cannot achieve adhesiveness comparable to that achieved by the above-described conventional treatment method using a mixed aqueous solution of sulfuric acid and oxalic acid. Therefore, it cannot be said that a bonded structure obtained by bonding a stainless steel plate or the like has bond strength and durability high enough for practical use, and therefore the bonded structure cannot be stably used for a long period of time.
Further, Patent Literature 3 discloses a primer composition containing a multifunctional epoxy resin and a bisphenol A-type epoxy resin as main components and imidazole as a curing agent. However, the primer composition is poor in film-forming properties due to the absence of a filler. Further, the primer composition is diluted with an organic solvent such as toluene or methyl ethyl ketone to secure coating workability, and therefore it is necessary to take measures against air pollution caused by VOCs (Volatile Organic Compounds), which is disadvantageous in that many restrictions are imposed during production and use.
Using a silane coupling agent is known as a technique for imparting adhesiveness to common steel plates, stainless steel plates, aluminum plates, aluminum alloy plates, etc. For example, Non-Patent Literature 1 describes that the adhesiveness of stainless steel plates, aluminum plates, aluminum alloy plates, etc. is improved by treatment using a silane coupling agent having a functional group reactive with an epoxy-based adhesive. However, the silane coupling agent has a monomer structure, and therefore it is difficult to uniformly apply its diluted solution onto a soiled metal surface in practice.
That is, the silane coupling agent can be tightly attached to an unsoiled metal surface by the condensation reaction of, for example, an alkoxy group or its hydrolysate, i.e., a silanol group and a hydroxyl group in an oxide layer on the metal surface. However, a metal surface is usually contaminated with organic or inorganic matter present in the atmosphere, and the organic or inorganic matter deposited on an oxide layer on the metal surface is already tightly attached. It is actually difficult to completely remove such a contamination layer. Therefore, at present, it is difficult to uniformly attach the silane coupling agent to a soiled metal surface in practice.
Further, with the spread of the bonding method, there are more and more cases where adhesive joints are exposed to a severe usage environment such as a high-temperature and high-humidity environment. For example, in the case of immersion in boiling water, i.e., exposure to the severest conditions, it is difficult for surface treatment using a silane coupling agent or the like to suppress a reduction in the adhesiveness of adhesive joints. It is to be noted that Non-Patent Literature 2 proposes silicoater treatment as treatment for imparting stable adhesiveness to adhesive joints of various metal materials even in boiling water. However, the silicoater treatment is high-temperature flame treatment, and therefore its application is limited to small base materials.